Solidifying meterial for cell electrolyte solution, and cell comprising the solidifying material

ABSTRACT

A solidifying material for a cell electrolyte solution is a block copolymer, which comprises, as segments A, a polymer non-compatible with the cell electrolyte solution and, as segments B, a polymer compatible with the cell electrolyte solution. The solidifying material absorbs and solidifies the cell electrolyte solution. A smallest unit of the block copolymer is A-B-A. To each of the segments B, at least one group selected from the group consisting of a carboxyl group, an ester group, a hydroxyl group, a sulfonic group, an amino group, a cyclic carbonate group and a polyoxyalkylene group is bonded via a −S— bond or a —C— bond.

BACKGROUND OF THE INVENTION

[0001] a) Field of the Invention

[0002] This invention relates to a solidifying material for cell orbattery (hereinafter collectively called “cell”) electrolyte solutionand a cell comprising the solidifying material as a constituent element.The term “cell electrolyte solution” may hereinafter be referred tosimply as an “electrolyte solution”, and the term “solidifying materialfor an electrolyte solution” may hereinafter be referred to simply as“solidifying material”.

[0003] b) Description of the Related Art

[0004] As a cell electrolyte is conventionally in a liquid form, it issealed in a case from the standpoint of safety. To safely hold theelectrolyte solution over a long time, the case is required to bestrongly built. As a result, it has heretofore been difficult to form acell into a thin structure. It has

SUMMARY OF THE INVENTION

[0005] The conventional high molecular substances for solidifyingelectrolyte solutions have crosslinked structures, are insoluble insolvents, and do not melt under heat. Accordingly, they cannot be formedinto thin films of uniform thickness. Use of a solid electrolyte in theform of a thin film is indispensable for the construction of a cell ofsmaller dimensions, especially of a reduced thickness. Because theabove-described high molecular substances cannot be formed into thinfilms, it has heretofore been difficult to obtain a solid electrolyte inthe form of a thin film of uniform thickness.

[0006] An object of the present invention is, therefore, to provide asolidifying material for a cell electrolyte solution, which can beformed in to a thin film or sheet (which may herein after becollectively called “film”) of uniform thickness and can easily absorband solidify the electrolyte solution.

[0007] Another object of the present invention is to provide anon-compatible with the cell electrolyte solution and, as segments B, apolymer compatible with the cell electrolyte solution, and absorbs andsolidifies the cell electrolyte solution, a smallest unit of the blockcopolymer is A-B-A, and to each of the segments B, at least one groupselected from the group consisting of a carboxyl group, an ester group,a hydroxyl group, a sulfonic group, an amino group, a cyclic carbonategroup and a polyoxyalkylene group is bonded via a —S— bond or a —C—bond; and a cell comprising the solidifying material as a constituentelement.

[0008] The present invention, in a second aspect thereof, also providesa solidifying material for a cell electrolyte solution, characterized inthat the solidifying material is a graft copolymer comprising, assegments A, a polymer non-compatible with the cell electrolyte solutionand, as segments B, a polymer compatible with the cell electrolytesolution, and absorbs and solidifies the cell electrolyte solution, andto each of the segments B, at least one group selected from the groupconsisting or sheet of a polymer having properties that the polymer isinsoluble in the cell electrolyte solution but the polymer absorbs andsolidifies the cell electrolyte solution, and a backing reinforcing thefilm or sheet, and the backing is a woven fabric, a nonwoven fabric or aporous film; and a cell comprising the solidifying material as aconstituent element.

[0009] The solidifying materials according to the present invention canbe dissolved or finely dispersed in appropriate solvents or can becaused to melt by heat, so that they can be formed into films each ofwhich has a desired thickness. Namely, the solidifying materialsaccording to the present invention can be formed in to thin films ofuniform thickness, and can easily absorb and solidify cell electrolytesolutions. As these films can be provided with enhanced strength byreinforcing them with backings, these films can each be formed with astill reduced thickness. These film-shaped solidifying materials canconveniently absorb and solidify electrolyte solutions, and thethus-solidified electrolyte solutions have good electrical because thewoven fabric has adequate strength despite its large opening area andmoreover, a solidifying material having a large particle size can alsobe used for the preparation of a coating formulation which is useful forforming a film.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0010] (First Aspect of the Present Invention)

[0011] The solidifying material according to the first aspect of thepresent invention is characterized in that the solidifying material is ablock copolymer comprising, as segments A, a polymer non-compatible withthe cell electrolyte solution and, as segments B, a polymer compatiblewith the cell electrolyte solution, and absorbs and solidifies the cellelectrolyte solution; a smallest unit of the block copolymer is A-B-A;and to each of the segments B, at least one group selected from thegroup consisting of a carboxyl group, an ester group, a hydroxyl group,a sulfonic group, an amino group, a cyclic carbonate group Elastomers(Special Edition—Thermoplastic Elastomers”, 24 (12) 1976 and SekiyuGakkai Shi (Bulletin of the Japan Petroleum Institute), 18, 565 (1975).These block copolymers are high molecular substances each of which has astructure such as (Segment A)-(Segment B)-(Segment A) that the segmentB, which has an unsaturated double bond, is flanked at two pointsthereof between the segments A, as expressed under the name of theso-called tele-block copolymer type, multi-block copolymer type orstar-shaped block copolymer type. Further, a single-block copolymercomposed of segments A and segments B may also be mixed in these highmolecular substances. Preferably, each of these high molecularsubstances has a weight average molecular weight of from 10,000 to500,000.

[0012] As the segments A which constitute the solidifying materialaccording to the first aspect of the present invention, a polymerselected from polystyrene, polyethylene or polypropylene is preferred.As the segments B, on the other hand, a polymer selected frompolybutadiene, polychloroprene or polyisoprene preferably be in a rangeof from 0.5 to 70 wt. %. A content lower than 0.5 wt. % is too low toexhibit the crystallization effect of the segments A for the copolymer.A content higher than 70 wt. %, on the other hand, results in asolidifying material having a smaller liquid absorption rate for theelectrolyte solution. The preferred content is in a range of from 1.0 to50 wt. %.

[0013] The segments B which also constitute the solidifying materialaccording to the first aspect of the present invention is a polymerselected from the group consisting of polybutadiene, polychloroprene andpolyisoprene, and the polymer can preferably have a weight averagemolecular weight of from 10,000to 300,000. The content of the segments Bin the block copolymer may be 99.0 to 50 wt. %, preferably 95.5 to 30wt. %.

[0014] Each segment B has a group, which is compatible with theelectrolyte solution, via a —S— bond or a —C— bond. Examples of thecompatible group can include a carboxyl group, ester groups, a hydroxylgroup, a sulfonic group, an amino group, cyclic carbonate groups, andether groups. Illustrative of the ether systems. It is preferred toselect such compatible groups as permitting absorption of a solution ofone of these systems and to introduce them into the segments B.

[0015] As an illustrative method for the introduction of theabove-described compatible groups into the segments B, a compatiblecompound containing one mercapto group (—SH), acid sodium sulfite(sodium hydrogensulfite) or maleic anhydride is added to double bonds inthe segments B. Examples of the mercapto-containing compound can includethioglycolic acid, thiolactic acid, thiomalic acid, thiosuccinic acid,thiosalicylic acid, mercaptopropane-sulfonic acid, thioethanolamine,thioglycol, and thioglycerin. In the presence of a free radicalgenerator, for example, azobisisobutyronitrile, azobiscyanovaleric acid,benzoyl peroxide, lauroyl peroxide, cumene hydroperoxide, ammoniumpersulfate or an alkali salt thereof, or hydrogen peroxide, or by simplyheating, the mercapto compound, maleic anhydride or acid sodium sulfiteis added to the segments B to obtain the oxide or propylene oxide to thegroups so introduced. The polyethylene oxide groups or polypropylenegroups so added may preferably have a weight average molecular weight ina range of from 200 to 1,000.

[0016] Upon introduction of the compatible groups, it is preferred toconduct the introduction by using a solvent. Preferred examples of thesolvent can include cyclohexane, methylcyclohexane, toluene, xylol,terpene, pentane, naphthene, kerosene, methyl ethyl ketone, acetone,tetrahydrofuran, dimethylformaldehyde, dioxolane, dioxane,ethylcellosolve, diethylcellosolve, ethyl acetate, propyl acetate, butylacetate, butyl alcohol, propyl alcohol, isopropyl alcohol, ethylalcohol, methanol, and water.

[0017] The solidifying material according to the first aspect of thepresent invention obtained as described above can take any form,including a form in which the solidifying material is dissolved in anaqueous system, including a form in which the solidifying material isdispersed in water, a form in which of the present invention ischaracterized in that the solidifying material is a graft copolymercomprising, as segments A, a polymer non-compatible with the cellelectrolyte solution and, as segments B, a polymer compatible with thecell electrolyte solution, and absorbs and solidifies the cellelectrolyte solution; and to each of the segments B, at least one groupselected from the group consisting of a carboxyl group, an ester group,a hydroxyl group, a sulfonic group, an amino group, a cyclic carbonategroup and a polyoxyalkylene group is bonded.

[0018] Illustrative of the segments A are polystyrene, polyethylene,polypropylene, poly(meth)acrylate esters and polyacrylonitrile, each ofwhich has a weight average molecular weight of form 3,000 to 20,000 andcontains an α, β-ethylenically unsaturated group at an end thereof. Aweight average molecular weight lower than 3,000 is too low to make thesegments A exhibit their crystallization-dependent, physicalcrosslinking effect in the graft copolymer. A weight average molecularweight higher than 20,000, on the other hand, makes it difficult toproduct range of form 2.5 to 50 wt. %.

[0019] Examples of a monomer, which has a group compatible with theelectrolyte solution and is to be graft-copolymerized with the segmentsA, can include (meth)acrylic acid, maleic acid, vinylbenzoic acid,(meta)styrenesulfonic acid, 2-acryloylamido-2-methyl-1-propanesulfonicacid, methacryloxypropylsulfonic acid, vinylsulfonic acid, alkali metalsalts such as polyoxyethylene alkyl ether sulfosuccinic acid or alkalinemetal salts thereof, 4-vinylpyridine, 2-vinylpyridine,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,(2-oxo-1,3-dioxolan-4-yl)methyl (meth)acrylate, 2-hydroxyethyl(meth)acrylate, and 2-hydroxypropyl (meth)acrylate.

[0020] From these monomers, a preferred monomer is selected dependingupon the electrolyte solution. Electrolyte solutions include bothaqueous and non-aqueous systems. It is preferred to graft-polymerizesuch a monomer as permitting absorption of group. Illustrative of such amonomeric ester are methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, lauroyl(meth)acrylate, stearyl (meth)acrylate, acrylonitrile, styrene, vinylacetate, (2-oxo-1,3-dioxolan-4-yl)methyl (meth)acrylate,(meth)acryloyl-containing polyethylene glycol (weight average molecularweight: 200 to 1,000), (meth)acryloyl-containing polypropylene glycol(weight average molecular weight: 200 to 1,000), and(meth)acryloyl-containing polyethylene glycol/polypropylene glycolcopolymer (weight average molecular weight: 200 to 1,000).

[0021] Among these, monomers important for the formation of segments B,which are suited for the transfer of ions in a non-aqueous electrolyteemployed in cells, are monomers containing polyoxyalkylene groups whichinclude at least a polyethylene glycol group. Use of a monomer, whichcontains a polyethylene glycol group as is or contains a copolymer ofethylene oxide and propylene oxide, is preferred. polyethylene glycoldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, tripropylene glycoldi(meth)acrylate, hydroxypivalate ester neopentyl glycoldi(meth)acrylate, trimethylol propane tri(meth)acrylate, pentaerythritoltri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate. Thesepolyfunctional monomers can be added preferably in a proportion of 5 wt.% or less of the above-mentioned monofunctional monomer.

[0022] As a polymerization initiator usable upon graft copolymerization,the same polymerization initiator as that described above in connectionwith the first aspect of the present invention can also be used.Further, as a solvent usable upon graft copolymerization, the samesolvent as that described above in connection with the first aspect ofthe present invention can also be used.

[0023] The solidifying material according to the second aspect of thepresent invention obtained as described above can take any form,including a form in which the solidifying material Mfg. Co., Ltd.

[0024] The solidifying material according to each of the first andsecond aspects of the present invention may preferably be in the form ofa film. Examples of a film-forming process can include the castingprocess in which a solution or dispersion of the solidifying material iscast and dried, the extrusion process in which the solidifying materialin a powdery form is dispersed in a thermoplastic resin and theresulting dispersion is extruded, and a process in which such adispersion is formed into a film by calendering. Especially in order toimpart excellent strength to a film to be obtained, a natural orsynthetic resin insoluble in the electrolyte can be added to thesolution or powder of the solidifying material.

[0025] Illustrative of the natural or synthetic resin are naturalrubber, and synthetic rubbers such as chloroprene, isoprene, butylrubber, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer,and hydrogenation products thereof. These copolymers can each be of anyone of bonding chemical resistance is effective.

[0026] The thickness of each film obtained as described above is 0.0001to 2 mm. A thickness smaller than 0.0001 mm involves a potential problemin that a homogeneous film may not be obtained. A thickness greater than2mm, on the other hand, makes it difficult to form the solidifyingmaterial into a film and, even if such a film is obtained, a long timeis needed for the absorption of the electrolyte. Moreover, such a greatthickness cannot provide a thin cell.

[0027] (Third Aspect of the Present Invention)

[0028] The solidifying material according to the third aspect of thepresent invention is characterized in that the solidifying materialcomprises a film or sheet of a polymer having properties the thatpolymer is insoluble in the cell electrolyte solution but the polymerabsorbs and solidifies the cell electrolyte solution, and a backingreinforcing the film or sheet; and the backing is a woven fabric, anonwoven fabric or a porous film. Preferred examples of theabove-described solidifying material particular limitation is imposedthereon. Illustrative are starch-based graft copolymers such as ahydrolysis product of starch-acrylonitrile graft copolymer,starch-acrylic acid graft copolymer, starch-styrenesulfonic acid graftcopolymer, starch-vinylsulfonic acid graft copolymer, andstarch-acrylamide copolymer; cellulose derivatives such ascellulose-acrylonitrile graft copolymer, cellulose-styrene-sulfonic acidgraft copolymer, and a crosslinked product of carboxymethylcellulose;hyaluronic acid, agarose, and collagen; polyvinyl alcohol derivatives,such as crosslinked polyvinyl alcohol polymer and polyvinyl alcoholsupersorbent gel/elastomer; crosslinked polyacrylic acid polymer, sodiumacrylate-vinyl alcohol copolymer, saponified product ofpolyacrylonitrile polymer, hydroxyethyl methacrylate polymer, maleicanhydride (co)polymers, vinylpyrrolidone (co) polymers, crosslinkedpolyethylene glycol-diacrylate polymer, crosslinked polypropyleneglycol-diacrylate polymer, ester-base polymers, amide-based appropriatedispersant in a non-aqueous medium.

[0029] In cadmium-nickel cells or nickel-hydrogen secondary cells,uncrosslinked copolymers each of which has been obtained using acrylicacid (or an acrylate salt), acrylamide or the like as a principalcomponent and has a weight average molecular weight of from 50,000 to1,000,000 can be used in place of the above-described (co)polymers.

[0030] In lithium cells as typical examples of those making use ofnon-aqueous electrolyte solutions, polymers obtained by copolymerizingmonomeric esters or the like to the above-described polymers can beused. Examples of such monomeric esters can include methyl(.meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl(meth)acrylate, tert-butyl (meth)acrylate, lauroyl (meth)acrylate,stearyl (meth)acrylate, acrylonitrile, styrene, and vinyl acetate. Tostrengthen the solidifying material which has swollen as a result ofabsorption of the electrolyte solution, the above-describedpolyfunctional monomer may be copolymerized peroxide. It is alsopossible to conduct the polymerization in the presence or absence ofsuch a polymerization catalyst, under irradiation of ultraviolet rays,electron beams or radiation, or under heat.

[0031] When the electrolyte is an aqueous solution, the solidifyingmaterial may preferably contain ion-compatible groups. When theelectrolyte is a non-aqueous solution, it is important for thesolidifying material to contain polyethylene oxide groups which takepart in the transfer of alkali ions. The amount of the electrolyte to beabsorbed in the solidifying material can be in a range of from 5 to5,000 wt. % based on the solidifying material. An absorption smallerthan 5 wt. % cannot provide the solidified electrolyte solution withsufficient electrical conductivity, while an absorption greater than1,000 wt. % results in swollen gel (the solidifying material in aswollen form as a result of absorption of the electrolyte solution) ofconsiderably reduced strength.

[0032] Among the above-described solidifying materials, the segments A,feature good dispersibility in other polymers having no compatibilitywith the electrolyte solution, and their particle sizes can be easilyreduced to several 100 nm to 10 μm.

[0033] The solidifying material according to the third aspect of thepresent invention is composed of the above-described solidifyingmaterial and the reinforcing backing. To improve the formability of thesolidifying material and the strength of the thus-formed product, it ispreferred to add, to the solidifying material, a polymer havingelastomeric property but no compatibility with the electrolyte solution.Such a polymer can be any one of the natural and synthetic rubbersdescribed above in connection with the first and second aspects of thepresent invention.

[0034] To reinforce the solidifying material, a woven fabric, a nonwovenfabric or a porous film can be used as a backing. The materials of thesebackings are, for example, polyethylene, polypropylene, polyamides,polyacrylonitrile, polyesters, a woven fabric or nonwoven fabric ofpolyacrylonitrile fibers at surfaces thereof with concentrated sulfuricacid or the like to introduce carboxyl groups therein. It is sufficientto apply such treatment only to fiber surfaces.

[0035] The woven fabric, nonwoven fabric or porous film as the backingmay preferably have a thickness in a range of from 1 to 1,200 μm, morepreferably from 2to 400 μm. A thickness smaller than 1 μm makes itdifficult to produce such a woven fabric, nonwoven fabric or porousfilm, while a thickness greater than 1,200 μm makes it difficult to forma thin, film-shaped solidifying material. The opening percentage of thenonwoven fabric may preferably in a range of from 95 to 10%. An openingpercentage higher than 95% bring about only small reinforcing effect forthe solidifying material, while an opening percentage lower than 10%leads to a film of extremely low electrical conductivity afterabsorption and solidification of the electrolyte. No particularlimitation is imposed on the type of weave of the woven fabric, andexamples of the weave can include backing by a gravure coater, a comma(knife) coater, a reverse coater or a blade coater, and is then dried,(3) the solidifying material is formed into a film in a manner by aknown method, and the film is then bonded onto the reinforcing backing(for example, a cast film of the solidifying material is bonded underpressure through heated rolls or on a press. In some instances, thecoating formulation can be fixed on the reinforcing backing by coatingthe coating formulation onto the reinforcing backing, immersing thethus-coated reinforcing backing in a poor solvent to make the layer ofthe solidifying material porous, and then drying the reinforcing backingwith the resultant porous layer carried thereon.

[0036] No particular limitation is imposed on a process for having theelectrolyte solution absorbed in the solidifying material according tothe present invention. For example, it is possible to have theelectrolyte solution absorbed in the solidifying material in the form ofa film reinforced with the reinforcing backing. As an alternative, it isalso possible to add the solidifying material by bonding a backing oneach electrode of a cell, dipping the electrode in a solution of thesolidifying material with the backing bonded on the electrode or coatingthe solution onto the backing on the electrode, and then conductingdrying. This process is effective for improving the mutual contactbetween the electrode and the film, which has been formed by solidifyingthe electrolyte solution, at the interface therebetween.

[0037] Examples of a cell electrolyte to be absorbed in theabove-described solidifying materials according to the first to thirdaspects of the present invention can include dilute sulfuric acid,potassium chloride, zinc chloride, potassium hydroxide, and lithiumsalts such as lithium perchlorate, LiBF, LiPF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂and LiC(CF₃SO₂)₂.

[0038] Illustrative of a medium in the above-described electrolytesolution are water, ethylene carbonate, propylene carbonate, dimethylcarbonate, ethyl methyl carbonate, dimethyl carbonate, γ-butyrolactone,methyl formate. methyl acetate,

[0039] (First Aspect of the Present Invention)

EXAMPLE 1 (Production Example of a Solidifying Material A)

[0040] A block copolymer (15 parts) composed of polystyrene,polybutadiene and polystyrene (polystyrene content: 30%, weight averagemolecular weight: 100,000) was dissolved in a mixed solvent formed oftoluene (45 parts), cyclohexane (75 parts) and methyl ethyl ketone (35parts), and the resulting solution was heated to 70° C. under a nitrogengas stream. Into the solution, thioglycolic acid (20 parts) andazobisisobutyronitrile (0.3 part) were added, followed by an additionreaction for 12 hours. The reaction mixture was washed with a saturatedaqueous solution of Na₂SO₄ to extract off unreacted thioglycolic acidfrom the reaction mixture. A 15% solution of potassium hydroxide inmethanol was added to the thus-washed solution to convert carboxylgroups in the resultant solidifying material into potassium salts.

[0041] The solvent was then distilled off to adjust the solid content ofthe solution to 30%. As a result of an analysis of had a swelling indexof 3,000% in deionized water.

[0042] Test 1 (Hot Potassium Hydroxide Durability Test)

[0043] The above-described solidifying material A was placed in a 20%aqueous solution of potassium hydroxide (electrolyte solution) and wascontinuously left over at 80° C. for 3 months. The absorption of thepotassium hydroxide solution in the solidifying material A was 400%, andno changes were observed on the solidifying material A.

[0044] Test 2

[0045] The above-prepared solution of the solidifying material A, thesolid content of which was 30%, and a solution (solid concentration:20%) of a polystyrene-polybutadiene-polystyrene (SBR-TR) block copolymer(polystyrene content: 30%, weight average molecular weight: 100,000) intoluene/methyl ethyl ketone were mixed at the respective solid ratios(weight ratios) described in Table 1. The resultant liquid mixtures werecast and dried on glass plates to form films of about 100 μm inthickness, respectively. the liquid absorption rates were calculated. Onthe other hand, the electrical conductivities were determined as will bedescribed next. Samples of the film were immersed at 25° C. for 24 hoursin the 3 months in the solutions (the deionized water, and the10%aqueous solution of potassium chloride), respectively. The film sampleswere taken out of the solutions, and were sandwiched between platinumplates of 1 cm². Across the respective film samples, voltages of 6V wereapplied, respectively. From the resulting currents, the electricalconductivities were calculated. TABLE 1 Electrical Electricalconductivity conductivity of film with Absorption Absorption SolidifyingElectrical of film with aqueous rate of rate of aqueous material A/conductivity deionized solution of KCl deionized solution SBR-TR of dryfilm water absorbed absorbed water of KCl (weight ratio) (Ω⁻¹cm⁻¹)(Ω⁻¹cm⁻¹) (Ω⁻¹cm⁻¹) (wt. %) (wt. %)  0/100 0 0 0 0 0 25/75  1.2 × 10⁻⁷ 5.7 × 10⁻³ 5.8 × 10⁻³ 250 150 50/50 2.45 × 10⁻⁶ 1.29 × 10⁻³ 6.9 × 10⁻³450 200 75/25 6.94 × 10⁻⁵ 2.42 × 10⁻³ 4.36 × 10⁻²  700 250 100/0   7.8 ×10⁻⁴  5.1 × 10⁻² 7.7 × 10⁻² 3,000 300

[0046] It has been found from Table 1 that, when the content of thesolidifying material A is 25% or higher, films with the respectivesolutions absorbed therein show sufficient electrical conductivities.From these results, it is understood that the solidifying materialaccording to the present invention is useful as a solidifying materialfor electrolyte solutions in “CADNICA” cells (Ni—Cd cells) ornickel-hydrogen cells.

EXAMPLE 2 Production Example of a Solidifying Material B

[0047] A block copolymer (8 parts) composed ofpolystyrene-polybutadiene-polystyrene (polystyrene content: 30%, weightaverage molecular weight: 100,000) was dissolved in a mixed solventformed of a petroleum-base solvent (50 parts) and methyl ethyl ketone(80 parts), and the resulting solution was heated to 70° C. under anitrogen gas stream. Into the solution, thioglycerin (12 parts) andazobisisobutyronitrile (0.2 part) were added, followed by an additionreaction for 12hours. After completion of the reaction, the reactionmixture was washed with to be about 200 nm. The swelling index of asolidifying material B, which had been taken out of the solution, indeionized water was 2,000%.

[0048] Incidentally, the solidifying material B can also absorb othersolvents such as tetrahydrofuran, dimethylformamide and methyl ethylketone to about 500 to 1,000%. Therefore, the solidifying material B canalso be used as a solidifying material for lithium cell electrolytesolutions containing aprotic solvents.

[0049] (Second Aspect of the Present Invention)

EXAMPLE 3 Production Example of a Solidifying Material C

[0050] Acrylic acid (30 parts), polyethylene glycol monomethacrylate (70parts, weight average molecular weight: about 300) andmethacryloyl-containing polystyrene (30 parts, weight average molecularweight: about 6,000) were dissolved in a mixed solvent formed of methylethyl ketone (100 parts) and cyclohexane (180 parts).Azoisobutyronitrile (1.1 parts) was mixed with the solution, followed bypolymerization at 70° C. water was about 2,000% based on its weight.

[0051] Test 3

[0052] The above-prepared solution of the solidifying material C and asolution (solid concentration: 20%) of apolystyrene-polybutadiene-polystyrene (SBR-TR) block copolymer(polystyrene content: 30%, weight average molecular weight: 100,000) intoluene/methyl ethyl ketone were mixed at the respective weight ratios(solid ratios) described in Table 2. The resultant liquid mixtures wereformed by casting into films of about 100 μm in thickness, respectively.Measurements of their liquid absorption rates and electricalconductivities were conducted by the same methods as in Test 1. TABLE 2Electrical Electrical conductivity conductivity of film with AbsorptionAbsorption Solidifying Electrical of film with aqueous rate of rate ofaqueous material C/ conductivity deionized solution of KCl deionizedsolution SBR-TR of dry film water absorbed absorbed water of KCl (weightratio) (Ω⁻¹cm⁻¹) (Ω⁻¹cm⁻¹) (Ω⁻¹cm⁻¹) (wt. %) (wt. %)  0/100 0 0 0 0 025/75  1.2 × 10⁻¹¹ 1.1 × 10⁻³ 2.8 × 10⁻³ 250 160 50/50 1.0 × 10⁻⁶ 1.1 ×10⁻² 1.4 × 10⁻² 350 200 75/25 2.9 × 10⁻⁶ 2.2 × 10⁻² 8.4 × 10⁻² 510 220

[0053] It has been found from Table 2 that films, each of which containsthe solidifying material of the present invention at 25% or more andcontains an electrolyte solution absorbed therein, show sufficientelectrical conductivities. From these results, it is understood that thesolidifying material according to the present invention is useful as asolidifying material for electrolyte solutions in “CADNICA” cells (Ni—Cdcells) or nickel-hydrogen cells. Further, the solidifying materialaccording to the present invention can also absorb solvents other thanwater, such as tetrahydrofuran, dimethyl for amide and methyl ethylketone, at rates of from about 300 to 800%. Therefore, the solidifyingmaterial according to the present invention can also be used as asolidifying material for lithium cell electrolyte solutions containingaprotic solvents.

[0054] (Third Aspect of the Present Invention)

EXAMPLE 4 Production Example of a Solidifying Material D

[0055] Following the process disclosed in JP55-56615A, potassiumthioglycolate was added to 95 mole % of the double bonds of of thesolidifying material D in the solution was measured by the lightscattering method (Coulter A4 particle sizer) (this will apply equallyhereinafter). As a result, the average dispersed particle size was foundto be about 100 nm. The swelling index of the solidifying material D indeionized water was 100-fold.

EXAMPLE 5 Production Example of a Solidifying Material E

[0056] In a similar manner as described above, thioglycol was added to90 mole % of the double bonds of polybutadiene in a block copolymerpolystyrene-polybutadiene-polystyrene (polystyrene content: 30%, weightaverage molecular weight: 100,000), and ethylene oxide (7 moles onaverage) was added to the hydroxyl groups of the thioglycol to produce asolidifying material E. The solidifying material E was mixed with amixed solvent of toluene/cyclohexanone/MEK (35/35/30, weight ratio) toadjust the solid content to 20%. The average dispersed particle size ofthe solidifying material E in the solution was about 100 nm. Theswelling index of the solidifying material ratio) to adjust the solidcontent to 50%. The average dispersed particle size of the solidifyingmaterial F in the solution was about 300 nm. The swelling index of thesolidifying material F in deionized water was 30-fold.

EXAMPLE 7 Production Example of a Solidifying Material G

[0057] A crosslinked polymer composed of potassiumacrylate/N,N′-methylenebisacrylamide (99.5/0.5%) was produced as asolidifying material G by radical polymerization. The content ofwater-soluble components in the solidifying material G was 20%. Theswelling index of the solidifying material G in deionized water was200-fold.

EXAMPLE 8 Production Example of a Solidifying Material H

[0058] A commercial, crosslinked isobutylene/potassium maleate copolymer(isobutylene/potassium maleate=1/1, molar ratio) was provided as asolidifying material H. The swelling index of the solidifying material Hin deionized water was 320-fold.

EXAMPLE 9 Production Example of a Solidifying Material I

[0059] Crosslinked poly(N-vinylacetamide) obtained by radical of thesolidifying material J in deionized water was 1,000-fold.

EXAMPLE 11 Production Example of a Solidifying Material K

[0060] An acrylic acid (89.1%)/styrene (10%)/divinylbenzene (0.9%,purity: 55%) copolymer, which had been obtained by bulk polymerizationin the presence of azobutyronitrile as a polymerization initiator, wasneutralized with potassium hydroxide, dried, and then ground. Fineparticles of from 1 to 5 μm in particle size were provided as asolidifying material K. The swelling index of the solidifying material Kin deionized water was 130-fold.

[0061] The following reinforcing backings were provided:

[0062] (1) Woven fabric obtained by sulfonating the surfaces of apolypropylene fabric (thickness: 0.122 mm, basis weight: 33 g/m², threadthickness: 0.080 mm, opening diameter: 0.098 mm).

[0063] (2) Nonwoven fabric (A) obtained by treating a nonwovenpolyacrylonitrile fabric (thickness: 0.081 mm, basis weight: 45 g/m²)with sulfuric acid to hydrolyze fibers at the surfaces thereof andforming potassium salts. aromatic process oil were mixed at a weightratio of 64/21/15 with toluene to adjust the solid content to 20%. Acoating formulation of the solidifying material was obtainedaccordingly. The coating formulation was applied onto both sides of thewoven fabric (1), and then dried at 80° C. for 24 hours to obtain asolidifying film 1 of 0.11 mm in thickness (coat weight: 100 g/m²,weight basis; this will apply equally hereinafter).

EXAMPLE 13 Production Example of a Solidifying Film 2

[0064] The solidifying material F and the polystyrene-polybutadieneblock copolymer were mixed at a weight ratio of 75/25 with toluene toadjust the solid content to 20%. A coating formulation was obtainedaccordingly. The coating formulation was applied onto both sides of thewoven fabric (1), and then dried at 80° C for 24 hours to obtain asolidifying film 2 of 0.15 mm in thickness (coat weight: 100 g/m²).

EXAMPLE 14 Production Example of a Solidifying Film 3

[0065] The solidifying material G and the polystyrene-polybutadieneblock copolymer were mixed at a weight polystyrene-polybutadiene blockcopolymer were dispersed at a weight ratio of 70/30 in toluene by a“Dynomill” (high-speed bead mill) to adjust the solid content to 20%. Acoating formulation was obtained accordingly. The average dispersedparticle size of the solidifying material in the coating formulation wasabout 30 μm. The coating formulation was applied onto both sides of thewoven fabric (1), and then dried at 80° C. for 24 hours to obtain asolidifying film 4 of 0.2 mm in thickness (coat weight: 100 g/m²).

EXAMPLE 16 Production Example of a Solidifying Film 5

[0066] The solidifying material H and the polystyrene-polybutadieneblock copolymer were dispersed at a weight ratio of 70/30 intetrahydrofuran by a “Dynomill” (high-speed bead mill) to adjust thesolid content to 30%. A coating formulation was obtained accordingly.The average dispersed particle size of the solidifying material in thecoating formulation was about 25 μm. The coating formulation was appliedonto both sides of the woven fabric (1), and then coating formulationwas obtained accordingly. The average dispersed particle size of thesolidifying material in the coating formulation was about 35 μm. Thecoating formulation was applied onto both sides of the woven fabric (1),and then dried at 80° C. for 24 hours to obtain a solidifying film 6 of0.12 mm in thickness (coat weight: 100 g/m²).

EXAMPLE 18 Production Example of a Solidifying Film 7

[0067] The solidifying material D, the polystyrene-polybutadiene blockcopolymer and an aromatic process oil were mixed at a weight ratio of64/21/15 with toluene to adjust the solid content to 20%. A coatingformulation was obtained accordingly. The coating formulation wasapplied onto both sides of the woven fabric (A), and then dried at 80°C. for 24 hours to obtain a solidifying film 7 of 0.9 mm in thickness(coat weight: 40 g/m²).

EXAMPLE 19 Production Example of a Solidifying Film 8

[0068] The solidifying material D, the polystyrene-polybutadiene blockcopolymer and an aromatic

[0069] A solidifying film 9 (coat weight: 10 g/m²) was obtained in ansimilar manner as in Example 16 except that the nonwoven fabric (B) andthe solidifying material K were used instead of the woven fabric (1) andthe solidifying material H, respectively.

REFERENTIAL EXAMPLE 1 Production Example of a Solidifying Film 10

[0070] A cast film (solidifying film) 10 of 100 μm in thickness withoutthe woven fabric in Example 12 was produced.

REFERENTIAL EXAMPLE 2 Production Example of a Solidifying Film 11REFERENTIAL EXAMPLE 2 Production Example of a Solidifying Film 11

[0071] The solidifying material J and the polystyrene-polybutadieneblock copolymer were dispersed at a weight ratio of 70/30 in toluene bya “Dynomill” (high-speed bead mill) to adjust the solid content to 30%.A coating formulation was obtained accordingly. The average dispersed

[0072] The individual solidifying films of the above-described Examplesand Referential Examples were ranked in the following properties. Theresults are shown in Table 3.

[0073] (1) Strength of Solidifying Film

[0074] Using each film of 15 mm in width, its tensile strength wasmeasured at a tensile speed of 100 mm/min by a strength measuringmachine (“Strograph EL”, trade name; manufactured by Toyo SeikiSeisaku-sho, Ltd.) under an environment of 20° C. and 60% RH. Eachsample was measured 10 times, and an average of the measurement resultswas recorded as measurement data.

[0075] (2) Electrical Conductivity

[0076] Samples of each film were immersed at 20° C. for 24 hours in a10% aqueous solution of potassium chloride and deionized water,respectively, and were then taken out. The samples were each heldbetween two platinum plates of 1 cm². From currents produced uponapplication of voltages of 6V across the samples, respectively, theelectrical conductivities of the samples were calculated.

[0077] <When no backing was included>

Liquid absorption rate (%)=[(W ₁ −W ₀)/W ₀]×100

[0078] W₁: Weight of the film after liquid absorption (g/m²)

[0079] W₀: Weight of the film before liquid absorption (g/m²)

[0080] <When a backing was included>

Liquid absorption rate (%)=[(W ₁ −W _(s) −W ₀)/(W ₀ −W _(s))]×100

[0081] W₁: Weight of the film after liquid absorption (g/m²)

[0082] W₀: Weight of the film before liquid absorption (g/m²)

[0083] W_(s): Weight of the backing (g/m²)

[0084] (4) Surface Condition

[0085] Each coating formulation was applied onto a woven fabric ornonwoven fabric. The coated surface was visually observed. The surfacecondition was ranked in accordance with the following standards.

[0086] A: Extremely smooth and uniform.

[0087] B: Smooth and good uniformity.

[0088] C: Rugged, and coating was difficult. TABLE 3 Electricalconductivity Liquid absorption Film (Ω⁻¹m⁻¹) rate (%) Surface strengthKCl solution Water KCl solution Water condition Ex. 12 78 40 × 10⁻⁴ 10 ×10⁻⁴ 230 4,000 A Ex. 13 80.5 33 × 10⁻⁴  7 × 10⁻⁴ 200 800 A Ex. 14 101 80× 10⁻⁴ 45 × 10⁻⁴ 400 10,000 B Ex. 15 92 72 × 10⁻⁴ 35 × 10⁻⁴ 350 20,000 BEx. 16 70 35 × 10⁻⁴ 20 × 10⁻⁴ 270 10,000 B Ex. 17 15 48 × 10⁻⁴ 15 × 10⁻⁴300 1,500 A Ex. 18 25 20 × 10⁻⁴ 10 × 10⁻⁴ 150 1,000 A Ex. 19 120 35 ×10⁻⁴  7 × 10⁻⁴ 200 3,000 A Ex. 20 80 75 × 10⁻⁴ 50 × 10⁻⁴ 1,500 7,500 ARef. Ex. 1 2 57 × 10⁻⁴ 50 × 10⁻⁴ 200 3,500 A Ref. Ex. 2 100 0.1 × 10⁻⁴ Measurement 350 10,000 C (substantial was scattering of impossiblemeasurement data)

EXAMPLE 21 Production Example of Solidifying Film 12

[0089] The solidifying material E, the polystyrene-polybutadiene blockcopolymer, lithiumperchlorate, ethylene carbonate and propylenecarbonate were mixed at a weight ratio of 1/0.5/1/10/10 withtetrahydrofuran to adjust the solid content to 20%. A coatingformulation was obtained accordingly. The coating formulation wasapplied onto both sides of the woven fabric (A), and then dried at 60°C. for 48 hours to obtain a solidifying film 12 of 0.12 mm in thickness.The film 12 had an ion conductivity of 2.0×10⁻³ S/cm, and was by nomeans usable in a lithium cell.

1-9. (canceled)
 10. A solidifying material for a cell electrolytesolution, characterized in that said solidifying material comprises afilm or sheet of a polymer having properties that said polymer isinsoluble in said cell electrolyte solution but said polymer absorbs andsolidifies said cell electrolyte solution, and a backing reinforcingsaid film or sheet; and said backing is a woven fabric, a nonwovenfabric or a porous film.
 11. A solidifying material according to claim10, wherein said polymer is a block copolymer comprising, as segments A,a polymer non-compatible with said cell electrolyte solution and, assegments B, a polymer compatible with said cell electrolyte solution,and absorbs and solidifies said cell electrolyte solution; a smallestunit of said block copolymer is A-B-A; and to each of said segments B,at least one group selected from the group consisting of a carboxylgroup, an ester group, a hydroxyl group, a sulfonic group, an aminogroup, a cyclic carbonate group and a polyoxyalkylene group is bondedvia a —S— bond or a —C— bond or a graft copolymer comprising, assegments A, a polymer non-compatible with said cell electrolyte solutionand, as segments B, a polymer compatible with said cell electrolytesolution, and absorbs and solidifies said cell electrolyte solution; andto each of said segments B, at least one group selected from the groupconsisting of a carboxyl group, an ester group, a hydroxyl group, asulfonic group, an amino group, a cyclic carbonate group and apolyoxyalkylene group is bonded.
 12. A solidifying material according toclaim 10, wherein said polymer is a polymer which comprises, as aprincipal component, polyacrylic acid, poly(N-vinylacetamide),poly[(2-oxo-1,3-dioxoran-4-yl)methyl (meth)acrylate] or polyacrylamide.13. A solidifying material according to claim 10, which is in a form ofparticles having an average particles size not greater than 100 μm. 14.A solidifying material according to claim 10, wherein said backing ismade of polyethylene or polypropylene.
 15. A solidifying materialaccording to claim 10, wherein said backing is a film or sheet of from 1to 1,200 μm in thickness and of from 95 to 100% in porosity.
 16. Asolidifying material according to claim 10, further comprising notgreater than 85 wt. %, based on said polymer, of an elastomernon-compatible with said electrolyte solution.
 17. A cell comprising, asa constituent element, a solidifying material according to claim 10.